The candidate is a pediatric rheumatologist with advanced training in clinical epidemiology and a research focus on the musculoskeletal complications of juvenile rheumatoid arthritis (JRA). This application proposes a comprehensive, interdisciplinary program that will provide the candidate with the skills and experience necessary for his development into an independent investigator in pediatric rheumatology and epidemiology. His career development will be guided by an outstanding team of experienced investigators in nutrition, body composition, bone health, epidemiology, exercise physiology, and psychology. His training will include formal coursework in advanced methods in nutritional assessment, behavior prediction, and biostatistical techniques for the analysis of longitudinal data. During his rheumatology fellowship, he conducted a preliminary study of the functional muscle-bone unit in children with JRA, demonstrating substantial deficits in bone density and structure in strong association with decreased muscle mass. The research component described in this application proposes to extend this study to examine longitudinal changes in bone structure and muscle mass, and to identify determinants of cachexia in children with JRA, such as increased resting energy expenditure, inflammation and decreased physical activity. These studies will include advanced research methods, such as quantitative computed tomography to assess bone and muscle, the doubly labeled water technique to assess energy expenditure, and accelerometry to assess physical activity patterns. In addition, the study will incorporate measures of inflammation and regulators of osteoclastogenesis. This proposal includes a pilot and feasibility study to assess the adherence, safety, and efficacy of whole body vibration in children with JRA, an intervention designed to load the skeleton and improve bone mass. Completion of this study will provide the applicant with the data and experience needed to support subsequent independent proposals to improve musculoskeletal health in this high-risk population.